Reverse Linked List II

1. Title

2. Http address

https://leetcode.com/problems/reverse-linked-list-ii/

3. The question

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

4. My code(AC)

 1     //  Accepted
 2     public ListNode reverseBetween(ListNode head, int m, int n) {
 3          
 4         ListNode frontDump = new ListNode(-1);
 5         ListNode middleDump = new ListNode(-1);
 6         ListNode lastDump = new ListNode(-1);
 7         ListNode frontTail = frontDump;
 8         ListNode middleTail = null;
 9         ListNode tmp = null;
10         int len = 1;
11         frontDump.next = head;
12         ListNode p = head;
13         
14         while( p != null &&  len < m)
15         {
16             frontTail = p;
17             p = p.next;
18             len++;
19         }
20         frontTail.next = null;
21         
22         middleDump.next = p;
23         middleTail = p;
24         while( p != null && len <= n)
25         {
26             tmp = p.next;
27             p.next = middleDump.next;
28             middleDump.next = p;
29             p = tmp;
30             len++;
31         }
32         
33         lastDump.next = p;
34         
35         if( m == 1)
36         {
37             middleTail.next = lastDump.next;
38             return middleDump.next;
39         }
40         if( middleDump.next != null)
41         {
42             frontTail.next = middleDump.next;
43             middleTail.next = lastDump.next;
44         }else{
45             frontTail.next = lastDump.next;
46         }
47         return frontDump.next;
48     }